1. Field of the Invention
This invention generally relates to an optical disc system and in particular to a servo error detecting method in an optical disc system using an optical disc as a data storage device.
2. Description of the Prior Art
In general, an optical disc system uses an optical disc which can store a large amount of data, e.g., approximately 200-800 mega bytes in the case of an optical disc having a diameter of 5.25 inches. In such an optical disc, the track pitch is extremely small; it is of the order of 1.6 microns. And, in an optical disc drive, a fine tracking servo control operation is carried out so as to keep a laser beam accurately onto a single intended recording track of the optical disc. In order to carry out such a tracking control operation, an optical disc 3 includes a transparent substrate 1 having formed thereon a recording film 2 shown in FIG. 2a and guide grooves 4 are previously formed as spaced apart from each other in a radial direction to define a recording track between two adjacent ones of the guide grooves 4.
If a laser beam irradiated onto the optical disc 3 through an objective lens 5 is located at the center of a track T as indicated by a point A in FIG. 2a, i.e., at the center between the two adjacent guide grooves 4, a tracking error signal TrE will have a zero value as indicated by A in FIG. 2b. And, if the position of the laser beam spot on the optical disc 3 is shifted for example to a point B from the center of the track T, the tracking error signal TrE will have an amplitude value and a sign which depend on the magnitude of the shift and direction of the shift. A tracking servo operation is carried out so as to cause the tracking error signal to become zero. As shown in FIG. 1, a data format of the optical disc 3 is effected sector by sector, and each sector includes a gap section 6, an address section (including a flag section which contains sector management information) 7 and a data section 8.
When the user wishes to record data in a target sector of the optical disc 3 (write mode), it goes through the following process.
(1) PRE-READ (CHECK READ)
1-a) The correctness of the address of a target sector is checked by reading the address information of address section 7. PA0 1-b) The presence of absence of a tracking error within a target sector is checked by a tracking error signal TrE. PA0 A flag pit is written in the address section 7 of a target sector and data is written into the data section 8. PA0 Immediately after writing, the data is read out and it is checked whether or not the data has been written properly.
(2) WRITE
(3) VERIFY
However, since the detection of tracking error, which corresponds to 1-b) of (1) above, is carried out by detecting a fluctuation in the tracking error signal TrE shown in FIG. 2b, inconveniences would arise, for example, when one or more guide grooves 4 are missing.
Described in greater detail in this respect, checking of tracking error during pre-read mode prior to an actual writing operation is important. Because, if a guide groove 4 is missing, the laser beam could be shifted away from the target sector into an adjacent sector in the adjacent track as indicated by the two-dotted line locus L.sub.2 in FIG. 1 instead of following an intented locus indicated by L.sub.1. If a writing operation is carried out under the circumstances, the data would be overwritten into the adjacent sector to thereby destroy the data which has been previously written in the adjacent sector.
In accordance with a prior art tracking error technique, a pair of tracking error detecting threshold levels +TH and -TH above and below a ground level GND of the tracking error signal TrE are used as shown in FIG. 2b, and if the tracking error signal TrE exceeds either of these threshold levels +TH and -TH, i.e., at point B or D in FIG. 2b, it is determined that a tracking error has occurred, whereby it is determined not to use the current target sector during pre-read mode. However, in accordance with this tracking error detecting technique, if the guide groove 4, for example, at point C in FIG. 2a is missing (or produces an nonfluctuating TrE signal), the tracking error signal TrE will have a substantially flat portion as shown in FIG. 2c, though the laser beam is shifted into the adjacent sector in the adjacent track by following points in the order of A, B, C, D and E, so that this portion of the tracking error signal TrE would not exceed either of the thresholds +TH and -TH. As a result, even if the laser beam has been shifted into the such a tracking error is not detected.
Such absence of a guide groove 4 could actually take place due for example to defects in the optical disc 1 and drop out of an exposure laser, such as Ar laser, during exposure of an original plate of disc stamper. The absence of a guide groove 4 is disadvantageous because it cannot detect off-tracking and it could result in overwriting of data in an undesired sector.
During reproduction, if servo signals, such as a tracking error signal and a focusing error signal, exceed a predetermined threshold level, a servo error condition is immediately determined and the reproduction operation is halted. However, even if a focusing error or tracking error condition has been detected after writing data, it might result from a defect on the optical disc 3 and not from any real focusing or tracking error condition. In such a case, if a servo error condition is immediately determined, there will be lost a chance to read data from the sector in question.